Not applicable.
Not applicable.
1. Field of the Invention
The present invention is directed to induction well logging. More particularly, the invention relates to a correction method for induction well logging measurements.
2. Background of the Invention
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as xe2x80x9clogging,xe2x80x9d can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or xe2x80x9csondexe2x80x9d is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or xe2x80x9cwireline.xe2x80x9d The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth""s formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
The sensors used in a wireline sonde usually include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
For a formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well-known physical characteristics. One characteristic is that the formation has a certain measurable resistivity (or conductivity), which can be determined by inducing an alternating electromagnetic field into the formation by a transmitter coil arrangement. The electromagnetic field induces alternating electric (or eddy) currents in the formation in paths that are substantially coaxial with the transmitter. These currents in turn create a secondary electromagnetic field in the medium, inducing an alternating voltage at the receiver coil. If the current in the transmitter coil is kept constant, the eddy current intensity is proportional to the conductivity of the formation. Consequently, the conductivity of the formation determines the intensity of the secondary electromagnetic field, and thus, the amplitude of the voltage at the receiver coil.
A number of different induction tools are known in the art. One known induction tool is the xe2x80x9chigh resolution array induction toolxe2x80x9d or HRAI as taught in U.S. Ser. No. 09/460,553, U.S. Pat. No. 6,606,565, filed Dec. 14, 1999, which is hereby incorporated by reference for all purposes. This is an array induction tool, operating with two frequencies and ten subarrays of six characteristic spacings. HRAI raw measurements are processed through skin-effect correction, borehole correction and software focusing to provide logs of six depth of investigation (10xe2x80x3, 20xe2x80x3, 30xe2x80x3, 60xe2x80x3, 90xe2x80x3, and 120xe2x80x3) with three vertical resolutions (1 ft, 2 ft and 4 ft). The use of this tool is not a limitation on the invention, however.
FIG. 1 shows an induction well logging instrument 10 disposed in a wellbore 2 drilled through earth formations. The earth formations are shown generally at 6, 7, 8, 9, 12, 13 and 14. The instrument 10 is typically lowered into the wellbore 2 at one end of an armored electrical cable 22 by means of a winch 28 or similar device known in the art.
The instrument 10 can include a telemetry/signal processing unit 20 (SPU).
The SPU 20 can include a source of alternating current (not shown separately). The alternating current is generally conducted through a transmitter 16 disposed on the instrument 10. Receivers 18A-18F can be disposed at axially spaced apart locations along the instrument 10. The SPU 20 can include receiver circuits (not shown separately) connected to the receivers 18A-18F for detecting voltages induced in each of the receivers 18A-18F. The SPU 20 can also impart signals to the cable 22 corresponding to the magnitude of the voltages induced in each of the receivers 18A-18F. It is to be understood that the number of transmitters and receivers, and the relative geometry of the transmitter 16 and the receivers 18A-18F shown in the instrument in FIG. 1 is not meant to be a limitation on the invention.
As is understood by those skilled in the art, the alternating current passing through the transmitter 16 induces eddy currents in the earth formations 6, 7, 8, 9, 12, 13, 14. The eddy currents correspond in magnitude both to the electrical conductivity of the earth formations 6, 7, 8, 9, 12, 13, 14 and to the relative position of the particular earth formation with respect to the transmitter 16. The eddy currents in turn induce voltages in the receivers 18A-18F, the magnitude of which depends on both the eddy current magnitude and the relative position of the earth formation with respect to the individual receiver 18A-18F.
The signals, corresponding to the voltages induced in each receiver 18A-18F, can be transmitted along the cable 22 to surface electronics 24. The surface electronics 24 can include detectors (not shown separately) for interpreting the signals transmitted from the instrument 10, and a computer 26 to perform the process according to the present invention on the signals transmitted thereto. It is to be understood that the SPU 20 could also be programmed to perform the process of the present invention.
The voltages induced in each receiver 18A-18F correspond to apparent electrical conductivity of all of the media surrounding the instrument 10. The media comprise the earth formations 6, 7, 8, 9, 12, 13, 14 and the drilling mud 4 in the wellbore 2. The degree of correspondence between the voltages induced in a particular receiver, and the electrical conductivity of the particular earth formation axially disposed between the particular receiver and the transmitter 16, can depend on the relative inclination of the layers of the earth formations, such as formation 12, and the axis of the instrument 10.
The eddy currents induced by the transmitter coils tend to flow in circular paths that are coaxial with the transmitter coils. For a vertical borehole traversing horizontal formations, each line of current flow ideally remains in the same formation along its entire flow path, and never crosses a bed boundary. Thus, one simplifying assumption that is made in relating the receiver voltage measurements to the conductivity of the earth formations is that the ground loops are positioned entirely within a portion of the earth formation having substantially circumferentially uniform conductivity. This assumption fails in cases where layers of the earth formations are not perpendicular to, but are inclined with respect to, the axis of the wellbore (and consequently the axis of the instrument). A boundary separates two layers which can have different conductivities. When the ground loops cross one or more bed boundaries, errors are introduced into the tool response. This is known as the xe2x80x9cdipping effectxe2x80x9d.
The dipping effect is classified into two components: the charge component and the volumetric component. The charge component is caused by an electric charge buildup when the induced eddy currents flow across inclined formation interfaces. Quantitatively, the charge component depends on the inner product of the electric field vector and the directional derivative of the formation conductivity. The volumetric component is caused by the fact that eddy currents take paths through formations of different conductivities.
Another tool error is commonly known as the xe2x80x9cnonlinear shoulder effect.xe2x80x9d As the induction well tool (also referred to as an induction well instrument) traverses the wellbore it commonly approaches, crosses, and then passes bed boundaries between formation layers. While the induction tool is proximate these bed boundaries, a portion of the receiver response comes from the bed or beds adjacent the bed in which the tool lies, introducing error into the measurements. It has been established that a portion of this tool response error in the regions proximate bed boundaries is non-linear. This nonlinearity makes it difficult to evaluate exactly the response portion that is from the adjacent bed, leading to an incorrect evaluation of the conductivity of the bed of interest.
Thus, an induction tool at an angle to a formation bed produces a series of inaccurate measurements. The larger the dip angle, the less accurate is the measurement with depth. Further, the log includes polarization xe2x80x9chornsxe2x80x9d, which correspond to the charge effect.
The measurements from induction tools are used to create formation resistivity well logs. Formation resistivity well logs are commonly used to map subsurface geologic structures and to infer the fluid content within pore spaces of earth formations. Formation resistivity well logs include electromagnetic induction logs. Of course, if not corrected for, the dipping error and shoulder bed error made in the raw measurements are reflected by inaccuracies in the formation resistivity well logs.
There exist several pieces of literature that address the problem of correcting for dipping error and shoulder bed effect. However, much of the discussion is centered around solutions that are inaccurate, or slow, or complicated. Further, resistivity log interpretation techniques were originally developed for vertical wells. Most of the methods in the literature that attempt to correct for at least a portion of the dipping error and shoulder bed effect error are incompatible with, or at least unduly complicate, these previous resistivity log interpretation techniques.
Ideally, a logging system and/or method could be devised that would solve for one or both of the dip error and nonlinear shoulder effect by a simple, straightforward and fast technique. Such a technique could be used during, for example, logging while drilling to make more accurate steering decisions.
The problems noted above are solved in large part by a method that determines dip error based on modeling or describing tool response at a depth-of-interest both with and without dip. The difference between the two modeled responses is defined as the dip error.
Another method determines shoulder effect error based on modeling tool response at a depth-of-interest without dip and modeling tool response with the elimination of the linear component of the shoulder bed effect. The difference between the two modeled responses is the non-linear component of the shoulder bed effect.
These methods may also be integrated into a system including a induction logging tool and a processor.
The disclosed devices and methods comprise a combination of features and advantages which enable it to overcome the deficiencies of the prior art devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.